The present invention relates to a method of preparing graphite intercalation compounds and the resultant products.
Highly lamellar forms of graphite have found wide ranging industrial applicability because of their low thermal and electrical resistivity and their ability to enhance thermal and electrical conductivity when added to a low or non-conductive particulate material. Such graphite has particular utility in making seals and gaskets for high temperature applications. Further, when highly lamellar graphite is mixed with or dispersed in particulate which are non-conductive or partially electrically conductive, the thin platelets of graphite become interlaced between the base particles, thus providing a more conductive path and more uniform contact with the particles than could achieved using the same concentration of non-lamellar graphite.
Exfoliated or expanded lamellar graphite has similar enhanced characteristics and utility. Thermally exfoliated graphite (xe2x80x9cTEGxe2x80x9d) has an accordion-like configuration of separated, stacked lamellae. Like naturally occurring lamellar graphite, delaminated, exfoliated, expanded graphite xe2x80x9cwormsxe2x80x9d are also used for applications such as enhancing thermal or electrical conductivity in various matrices. For example, in the manufacture of alkaline electrolyte batteries, delaminated exfoliated flake graphite is used in the positive electrode active material. See, e.g., U.S. Pat. No. 5,482,798 to Mototani et al., which is incorporated herein by reference. If the flake graphite can be expanded in a manner to maximize its surface area for a given mass and be successfully delaminated, greater conductivity can be attained for the positive electrode. This results in an improved discharge performance, higher rate capabilities, and longer useful life for the battery. Simultaneously, the amount of graphite needed to produce the electrode can be decreased, permitting an increase in the amount of the active electrode material, MnO2.
Typically, lamellar graphite has been expanded by the intercalation of a compound into or between the interlayers of the crystal structure of the graphite. The graphite intercalation compound (xe2x80x9cGICxe2x80x9d) is then expanded to dramatically enlarge the spaces between the graphite interlayers. The intercalation of lamellar graphite has been studied in detail and described in numerous technical papers and patents. For example, Hirschvogel et al. U.S. Pat. No. 4,091,083 and Greinke et al. U.S. Pat. No. 4,895,713 disclose chemical intercalation methods that involve soaking graphite particles in a solution comprising an aqueous acid and an aqueous oxidizing agent.
In Hirschvogel et al., graphite particles are soaked in sulfuric acid, to which hydrogen peroxide is added. The reaction mixture is agitated by, e.g., stirring, to maintain the graphite particles in a dispersed state. The graphite is thus converted to graphite hydrogensulfate. The excess acid is separated and the residual acid in the solid product is removed by washing. In Greinke et al., graphite flakes are mixed with an intercalation solution comprising sulfuric acid, phosphoric acid, etc. with an oxidizer such as nitric acid, perchloric acid, chromic acid, hydrogen peroxide, etc. Liquid-solid blending techniques completely disperse the liquid intercalation solution through the solid graphite flake, with the intercalation solution being introduced to the graphite while it is being stirred. Blending or mixing is continued to completely disperse the solution throughout the flakes.
U.S. Pat. No. 4,350,576 to Watanabe et al., which is incorporated by reference herein, describes an electrochemical intercalation process using an electrolytic intercalation solution preferably comprising sulfuric acid (50% aqueous solution or more) or nitric acid (30% aqueous solution or more) in which the graphite is subjected to electrolysis in which the current density is preferably 50 mA/cm2 or less. The intercalated graphite is then dried and heated to 1,000xc2x0 C. to obtain an expanded graphite.
Thus, while it has been known how to intercalate graphite, as more uses for the material have been discovered, it has become desirable to produce such a graphite intercalation compound in commercial quantities in a more efficient and economic manner. This means that sufficiently large quantities (i.e.,  greater than 150 kg) can be intercalated in a reasonable period of time (i.e.,  less than 1 hr.), while providing an intercalated graphite that can be expanded to a high bulk density (i.e.,  greater than 200 ml/g). Further, safety and environmental concerns relating to the use of potentially hazardous acids and oxidizers and energy consumption must be minimized.
Accordingly, it is the object of the present invention to provide a safe, efficient, economic, and environmentally acceptable method for producing graphite intercalation compounds that can be expanded to a high bulk density.
This object, as well as others that will become apparent upon reference to the following drawings and detailed description, is provided by a method of preparing graphite intercalation compounds in which graphite particles are immersed in an aqueous electrolyte media comprising both an acid and an oxidizing agent. The immersed graphite particles are subjected to an anodic current and then removed from the electrolyte and rinsed with a solvent. The excess solvent and electrolyte is then removed from the graphite particles. In a preferred method, the electrolyte comprises between approximately 99 Vol. % and 50 Vol. % of 66 Wt. % H2SO4 and between approximately 1 Vol. % and 50 Vol. % of 40 Wt. % HNO3. The current density to which the immersed particles are subjected is between approximately 5 mA and 2A per gram of graphite, and the immersed graphite particles are subjected to this current for between approximately 1 and 180 minutes. Optionally, the oxidizing agent may be selected from the group consisting of CrO3, KmnO4, (NH4)2 SO4, PbO2, MnO2, MnO, H2O2, and HClO4, instead of HNO3. Further, the graphite particles may be placed in a plating barrel which is immersed in the electrolyte and rotated while the graphite particles are subjected to the current. The resultant intercalated graphite has an expansion volume of from between about 100 ml/g to 2000 ml/g when heated at 1000xc2x0 C. for from 1 second to 10 minutes.